Element Distribution and Its Induced Peritectic Reaction during Solidification of Ti-Al-Nb Alloys

The element distribution and the microstructures of directionally solidified ingots of Ti-45Al-8Nb and Ti-46Al-8Nb alloys were studied by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA) equipped with wavelength-dispersive X-ray spectroscope (WDS). At high solidification rates, e.g., more than 50 μm/s, the ingot solidified in columnar β dendrites, while at low solidification rates, e.g., less than 30 μm/s, the solidification path changed from initial β solidification to L + β→α peritectic solidification, forming cellular dendrites with the β phase matrix surrounded by the α phase. The difference of Ti content in dendritic arms and interdendritic regions was not pronounced. The composition segregation was mainly caused by the mutual conversion of Al and Nb contents. Therefore, it was difficult to distinguish the variation of Ti in microstructure by EPMA-WDS map and line profiles. The composition of the peritectic α phase was different from that of the α phase transformed directly from the β phase. The Al content of the former was about 1 at% higher than that of the latter, while the Nb content was about 1 at% lower. The change of solidification path in the final solidified part resulted from the more severe segregation caused by slow solidification.